1. Field of the Invention
The present invention relates to a vehicle suspension system and more particularly to a vehicle suspension having a variable suspension property.
2. Description of the Prior Art
Conventionally, a vehicle suspension system includes a suspension spring assembly and a shock absorber or an oleo damper which are located in combination between the vehicle body and each wheel. In one type of suspension system, the shock absorber includes a variable orifice and the orifice size is controlled to change the damping property of the shock absorber. Such a vehicle suspension system has a problem in that pitching movements are produced in the vehicle body under a deceleration or an acceleration force and rolling movements are produced under a centrifugal force which i produced in cornering operation.
In order to suppress such movements in the vehicle body, the Japanese patent publication No. 59-14365 proposes a vehicle suspension which includes a hydraulic cylinder located between the vehicle body and the wheel and a gas spring having a gas chamber and a hydraulic chamber. The hydraulic chamber of the gas spring is connected with the hydraulic cylinder through an orifice. According to the proposal by the Japanese patent publication, the level of the vehicle body from the ground is detected and the hydraulic pressure to the cylinder is controlled in accordance with the detection signal.
In the suspension system of this type, the operation of the system can be represented by the following formula: EQU F=kx+c dx/dt
where;
F is the load on the suspension system, PA1 x is the displacement of the suspension system, PA1 k is the spring coefficient, and PA1 c is the absorption factor.
In the European patent publication No. 0 114 757, there is disclosed a vehicle suspension system which includes a hydraulic cylinder and the pressure to the cylinder is controlled in accordance with a factor determined by the above formula. More specifically the value F and dx/dt are detected and a desired displacement of the cylinder is obtained based on these values. According to the proposal by the European patent application, the desired displacement of the cylinder is changed to thereby change the spring coefficient and/or the damping factor dependent on the external turbulence such as the loads from the road on which the vehicle is running.
It should however be noted that in the suspension system as proposed by the Japanese patent publication or by the European application it is impossible to adjust the suspension property precisely in accordance with the vehicle running condition and the road condition. For example, once the suspension system is adjusted for a rough road to provide a relatively soft suspension property, such property will be maintained even in a cornering opeation wherein a relatively hard suspension property is desired. To the contrary, once the suspension system is adjusted for a cornering to provide a relatively hard suspension property so that the rolling of the vehicle body is suppressed, the relatively hard suspension property is maintained even if there is a rough road condition.